1. Field of the Invention
This invention relates to a metal vapor laser tube, and particularly to a metal vapor laser tube of the hollow-cathode type.
2. Description of the Prior Art
A metal vapor laser tube generates laser oscillations making use of vapor of a metal which is enclosed in the metal vapor laser tube together with a rare gas.
An exemplary one of metal vapor laser tubes which are practically used at present is a so-called positive column type metal vapor laser tube which generates laser oscillations making use of a positive column portion of a discharge. More particularly, a positive column type metal vapor laser tube which employs cadmium and helium therein is known.
Such a positive column type metal vapor laser tube which employs cadmium and helium therein can oscillate continuously at an ultraviolet wavelength of 325 nm as well as a visible wavelength of 442 nm in a short wavelength zone. Due to the characteristic, the demand for such positive column type metal vapor laser tubes has increased in recent years, and metal vapor laser tubes of the positive column type are used in various fields as light sources, for example, for laser printers, fabrication of color reproduction plates, photo-plotters, production of a semiconductor laser device of the DFB type, and particle measuring instruments.
A metal vapor laser tube of a different type from such a positive column type metal vapor laser tube is also known in principle which is called a hollow-cathode type metal vapor laser tube which generates laser oscillations making use of a negative glow portion of a discharge. Of late, increasing attention is paid particularly to a hollow-cathode type metal vapor laser tube which employs helium and cadmium therein due to its characteristics that it can oscillate continuously not only at an ultraviolet wavelength of 325 nm and at a visible wavelength of 442 nm in the short wavelength zone but also at a visible wavelength of 636 nm in the red zone and at visible wavelengths of 538 nm and 533 nm in the green zone. Besides, it is possible to reduce noises about by one tenth comparing with a positive column type metal vapor laser tube.
Exemplary construction of such a conventional hollow-cathode type metal vapor laser tube which has been manufactured for trial so far as described above is illustrated in FIG. 7. Referring to FIG. 7, the hollow-cathode type metal vapor laser tube shown includes a glass enclosure or tube 91, a lot of anode members 92, a cathode member 93 having a lot of discharge openings 95 formed therein, a lot of working metal containers 94 formed on the glass enclosure 91, heaters 96 and 97, and a pair of mirrors 98A and 98B. The glass enclosure 91 has, for example, helium enclosed therein, and a working metal M which may be, for example, cadmium is contained in the working metal containers 94. The heaters 96 are provided around respective projecting tube portions P.sub.A for holding anode members 92 formed on the glass enclosure 91 while the heaters 97 are provided around respective projecting tube portions P.sub.M of the glass enclosure 91 which form the working metal containers 94.
Such a hollow-cathode type metal vapor laser tube of the construction described above, however, has following drawbacks:
(1) It is a drawback that, since the glass enclosure 91 is complicated in structure, glasswork with high skill is required, and consequently mass-production by mechanization cannot be attained and it is difficult to put it into practical use as an industrial manufacture.
In particular, a time consuming and skillful work is required to form, on an outer wall of the glass enclosure 91, a large number of projecting tube portions P.sub.A in which the anode members 92 are installed and another large number of projecting tube portions P.sub.M in which the working metal M is contained. Besides, a time consuming and skillful work is required to seal and secure such a large number of anode members 92 in a right condition to the projecting tube portions P.sub.A. After all, it is impossible to establish an industrial production system by mechanization, and accordingly it is difficult to manufacture a hollow-cathode type metal vapor laser tube.
(2) When a laser is oscillated, the glass enclosure 91 which is normally made of hard glass is heated to a considerably high temperature by the heaters 96 and 97, and the glass enclosure 91 and the cathode member 93 which is normally made of copper, titanium or iron are heated to a high temperature also by heat of a discharge. Thermal expansions of the glass enclosure 91 and the cathode member 93 thus cause a change in relative position between the positions of inner ends of the anode members 92 and the discharge openings 95 formed in the cathode member 93. The change in relative position particularly increases substantially in proportion to the length of the glass enclosure 91. Consequently, a negative glow portion may be readily changed transiently or partially into an arc discharge and it cannot maintain stable discharge. It is a problem, therefore, that after all a function of the hollow-cathode type metal vapor laser tube as such is deteriorated.
It is to be noted that the discharge openings 95 should have an appropriate range in size and when the size of them is excessively great, the gain in laser oscillation is reduced. Accordingly, it is impossible in practical use to provide the discharge openings 95 of such a large diameter that can absorb a change in such relative position as described above which arises from thermal expansions.